How I finally understood what a class is

Which I find to be true. Some may argue that a certain teacher doesn't know how to do his/her job, but maybe it's that the way he/she teaches is not compatible with the way your brain is "wired". I guess this is one of those things that is not just right or wrong.

Many things are open to interpretation, which may lead to the loss of valuable information.

So, today I want to explain the way the idea of a class clicked for me. Not just in a conceptual way, but also on why they are useful in our code:

One of the first things we learn when we start programming are variables and constants.

So, if we are given the task of printing on the console your name, age and height, you'll probably do something like this:

Note: I decided not to include using a class like Scanner to request input from the user to keep it as simple as possible.

Let's refactor our code step by step by focusing on one person at a time.

The Chris class

In OOP (Object Oriented Programming), classes are made to represent both state and behaviour with a high level of cohesion. In english: that means that the variables and methods of a given class are related to each other. If a method does not use any of the variables inside a class, it's probably a sign that it is where it doesn't belong.

Now that we have this idea, we can make a Chris class with a default constructor that will take the values of chrisName, chrisAge and chrisHeight:

A constructor is a special kind of function that must have the same name as the class it belongs to + have no return value (doesn't even need the void keyword), which is normally used to make sure that an instance of that class is in a valid state.

Valid state means that an object has the values that are expected.

An object is a concrete implementation of a class (we will see it in action later).

The keyword this is used to refer to the global scope of our class in order to differentiate the name parameter of the constructor from the actual global scope's name.

Now we can refactor our code in order to use our new and shiny Chris class!

So, instead of:

StringchrisName="Chris";intchrisAge=23;doublechrisHeight=1.85;

We can have:

Chrischris=newChris("Chris",23,1.85);

The keyword new is used to refer to the constructor of our Chris class.

And chris (notice the lower case "c") is what we call an object, because it is a concrete or "real" implementation of our Chris class.

But since we don't have our chrisName, chrisAge and chrisHeight anymore, our code won't compile correctly.

Interesting...

How can we fix this?

Well, if we go back to our Chris class implementation, we can see that our global scope's variables (also known as fields, private fields or member variables) are private. So, we can't access them from the outside.

In order to be able to do that, we must add public methods that can help us access that data.

Did you notice how those get...() methods don't use the this keyword? That's because there are no parameters that can match their names, so the compiler knows that we are referring to the global scope's variables instead.

Now that we have these methods, we can access them by using the dot (.) operator, like this:

What benefits did we get?

We reduced the noise of repeating both "Chris" and "Daniel" in more places than needed.

We managed to reuse our logic in a single class.

Now we can quickly know that both chris and daniel are concrete implementations of the same class, or how I like to call them: brothers.

Now we can use this Person class even in other projects and it will still work just fine.

Final words

I hope this example can help you, Mr./Mrs. Reader, to clear out your thoughts on what class are and why we use them in Object Oriented Programming :)

See you in the next post!

Bonus tip

This one comes from @alphashuro's comment down below: another benefit is that now we can replace the parts in our code where we print a Person's information by making a function that takes a Person object, like this:

And by making this "small" change we get the benefit of not having to maintain two different lines of code. There's just only 1 place in our code that we have to change in case we need to present someone's information in a different way.

Thanks to Alpha for bringing this up.

Now, there's another adjustment that I would do to our code.

Since the printPersonalInfo() function is only accepting Person objects, this means this method is directly dependent of the Person class. Which means, it should actually be part of it!

So let's go ahead and move our function to the Person class instead of having it hanging around inside our Main:

// JavapublicclassPerson{privateStringname;privateintage;privatedoubleheight;publicPerson(Stringname,intage,doubleheight){this.name=name;this.age=age;this.height;}// Imagine we still have the getters here :P// this is just to make the code block shorter.publicvoidprintInfo(){System.out.println("name: "+name+", age:"+age+", height:"+height);}}

Now, you may have noticed that I had to make a few adjustments:

Remove the person parameter.

Replace each Getter methods calls for the global variables.

Renamed the method from "printPersonalInfo()" to "printInfo()".

The last point can be optional or personal preference. Personally, I find the "Personal" part of the name to be little redundant since we know we will create a Person object later.

Ok, so now we also have to make some adjustments to our Main class with this new implementation:

The "prefix" variable pattern is one I mention often in the interviews I do. It's a strong indicator that there's a class hiding in your code: anytime you have multiple sets of variables with a common prefix, or suffix.

This is great, well done! I think an important benefit of Classes that can be added in the final section of this article is that you can create a method that takes a parameter of type Person and calls println using the parameter's getters, then you can use this function on each instance of Person, i.e. chris and daniel, without writing out println twice.

Nice post, well explained, i like it, if i would teach in a future some OO language i would go back to this post as a reference. I would like to add two little conceptual details.

In Java a default constructor is constructor that gets generated if you don't explicitly create one. That's the reason why you can create and no-param object without creating an explicitly constructor.

Finally, i would like to add that in the above example "chris" works as a reference of an object assigned to an object in memory.

Regarding the default constructor, I thought it would be a correct way of calling it since that implementation of the Person class does not allow the client of our code to create an object without passing any arguments to it. Would it be better to say "overriding the default constructor"?

Thanks to you, i really like it as an introduction for classes, you should continue doing this kind of post.

I considered can be called just param-constructor, if we explicitly add no-args-contructor can be called as a overloaded constructor. The Overrinding is not possible for constructors because the behavior strictly belongs to the class.

Christian -- terrific walkthrough of the practicalities of classes (as opposed to the abstract concept thereof that seems to be the canonical explanation for no good reason)!

Whenever I explain classes to the unacquainted, I find it useful to compare classes to a brand new data type that we, as users of any given language, get to create ourselves! This might be an imperfect analogy, depending on the language, but we all learn the basic primitive types when we first start (long before we get to groking classes). Later, we all develop some intuition about how those types behave. Ergo, a custom type (i.e. a class) is just a type which we get to direct!

Nicely explained! I am missing one thing though: the cohesion also implies a common lifetime of the objects. One of the things where behavior gets actually different from comparing classes vs. prefixed variables.